Rotary drag bits, reamers, and similar downhole tools for boring or forming holes in subterranean rock formations when drilling oil and natural gas wells drag discrete cutting structures, which use cutting elements referred to as “cutters,” mounted in fixed locations on body of the tool, against the formation by rotating the body of the tool. The rotation of the tool enables the cutters to fracture the formation through a shearing action, resulting in formation of small chips that are then evacuated hydraulically by drilling fluid pumped through carefully placed nozzles in the body of the tool.
One such fixed cutter, earth boring tool, generally referred to in the oil and gas exploration industry as a PDC bit, employs fixed cutters having a highly wear resistant cutting or wear surface comprised of a polycrystalline diamond compact (PDC) or similar highly wear resistant material. PDC cutters are typically made by forming a layer of polycrystalline diamond (PCD), sometimes called a crown or diamond table, on an erosion resistant substrate. The PDC wear surface is comprised of sintered polycrystalline diamond (either natural or synthetic) exhibiting diamond-to-diamond bonding. Polycrystalline cubic boron nitride, wurtzite boron nitride, aggregated diamond nanotubes (ADN) or other hard, crystalline materials are known substitutes and may be useful in some drilling applications. A compact is made by mixing a diamond grit material in powder form with one or more powdered metal catalysts and other materials, forming the mixture into a compact, and then sintering it with, typically, a tungsten carbide substrate using high heat and pressure or microwave heating. Sintered compacts of polycrystalline cubic boron nitride, wurtzite boron nitride, ADN and similar materials are, for the purposes of description contained below, equivalents to polycrystalline diamond compacts and, therefore, a reference to “PDC” in the detailed description should be construed, unless otherwise explicitly indicated or context does not allow, as a reference to a sintered compacts of polycrystalline diamond, cubic boron nitride, wurtzite boron nitride and other highly wear resistant materials. References to “PDC” are also intended to encompass sintered compacts of these materials with other materials or structure elements that might be used to improve its properties and cutting characteristics. Furthermore, PDC encompasses thermally stable varieties in which a metal catalyst has been partially or entirely removed after sintering.
Substrates for supporting a PDC wear surface or layer are typically made, at least in part, from cemented metal carbide, with tungsten carbide being the most common. Cemented metal carbide substrates are formed by sintering powdered metal carbide with a metal alloy binder. The composite of the PDC and the substrate can be fabricated in a number of different ways. It may also, for example, include transitional layers in which the metal carbide and diamond are mixed with other elements for improving bonding and reducing stress between the PCD and substrate.
Each PDC cutter is fabricated as a discrete piece, separate from the drill bit. Because of the processes used for fabricating them, the PCD layer and substrate typically have a cylindrical shape, with a relatively thin disk of PCD bonded to a taller or longer cylinder of substrate material. The resulting composite can be machined or milled to change its shape. However, the PCD layer and substrate are typically used in the cylindrical form in which they are made.
Fixed cutters are mounted on an exterior of the body of an earth boring tool in a predetermined pattern or layout. Furthermore, depending on the particular application, the cutters are typically arrayed along each of several blades, which are comprised of raised ridges formed on the body of the earth boring tool. In a PDC bit, for example, blades are generally arrayed in a radial fashion around the center axis (axis of rotation) of the bit. They typically, but do not always, curve in a direction opposite to that of the direction of rotation of the bit.
As an earth boring tool with fixed cutters is rotated, the cutters collectively present one or more predetermined cutting profiles to the earth formation, shearing the formation. A cutting profile is defined by the position and orientation of each of the cutters associated with it as they rotate through a plane extending from the earth boring tool's axis of rotation outwardly. A cutter's position along the cutting profile is primarily a function of its lateral displacement from the axis of rotation and not the particular blade on which it lies. Cutters adjacent to each other in a cutting profile are typically not next to each other on the same blade.
In addition to position or location on the bit, each cutter has an orientation. Generally, this orientation will be defined with respect to one of two coordinate frames: a coordinate frame of the bit, defined in reference to its axis of rotation; or a coordinate frame generally based on the cutter itself. The orientation of a cutter is usually specified in terms of a side inclination or rotation of the cutter and forward/back inclination or rotation of the cutter. Side inclination is typically specified in terms lateral rake or side rake angle, depending on the frame of reference used. Back inclination is specified in terms of an axial rake or back rake angle, depending on frame of reference used.